ASME VIII-1 design for a novice

[MotorMan12]

Working for a small company I at times have to perform duties well outside what I consider my core competence. This is such a case:

Part of a new structure we are building must be classed as a "pressure vessel". The geometry is simple, but due to size it is highly likely that the body and flanges are manufactured separately and joined by welding. , We have chosen ASME VIII-1 as the design code, even though it is not to be used in the US. No U-stamp is required, but a national authorizing agency will review the product and certify that it is as per the code. Our manufacturer is not experienced with supplying according to ASME VIII-1. It is, at this stage, not an option to go with another supplier. I have therefore examined the relevant sections of ASME VIII-1 (and other referenced sections) in order to understand what we must comply with, in order to follow this up properly.

Below I have outlined my understanding of what must be done. It would be very interesting to get a feedback on this understanding, as I appreciate that this is a field where a novice may well get into trouble.

1. Use material accepted by code: We have selected a material listed in ASME II, part D, table 1B: UNS C95400 (SB-148). It is accepted for ASME VIII-1 up to 600 F (well below max temp for this application)

2. Strength calculation according to code: Demonstrate that maximum stresses are below maximum allowable stress as given in ASME II, part D, table 1B (UG-24 (a)(3) allows for a correction factor of 0.9 with full radiography). We are bringing in outside help to produce a report demonstrating this.

3.Welding. Welding is new to me, but since ASME II, part D, Table 1B has a comment to UNS C95400 (SB-148) that it “must be post-weld heat-treated to 1150-1200F, …”, it leads me to believe that it can be welded. ASME IX, appendix E does not list this as a material for which an SWPS exists. As I understand it, this means that my manufacturer must obtain/make a WPS that is to be used by the (qualified) welder during the welding process. For my material with a P-number (35), this WPS could probably be picked from a "library" (there may be a standard ASME or ASTM WPS that could be used for this WPS?). The manufacturer in question must qualify this WPS by a PQR (which is specific to this manufacturer. It demonstrates that this manufacturer gets the right properties for the weld. Not sure if I have understood this correctly). If the above is correct, is this “a big deal”, or straight-forward for any foundry used to pouring such specialized material as Aluminum Bronzes?

4. NDT. The WPS specifies the type and extent of NDT to be performed.

5: PWHT. Although UNF-56 (a) states that nonferrous materials normally should not undergo PWHT, UNF-56 (c) states that this material is to be heated to 1150-1200F. Such a process should be standard for any normal manufacturer.

Any comments? What do I need to be concerned about?

 

[trottiey]

You're doing better than many bootstrappers I've seen. But you should beware that building a good safe pressure vessel is a specialized field that you cannot learn from an internet forum. Your company and your manufacturer needs to send its people on recognized courses before you undertake this; courses in other countries (like the US) if you have to. This is a matter of life or death for the people who will be working near your pressure vessel. Here are some of the things you might learn:

1) In addition to the WPS and PQR, the manufacturer also needs Welder Performance Qualifications (WPQ's) for each of the welders.

2) Passing the PQR and WPQ are a matter of equipment quality, welder skill, and company know-how. Someone who knows how to melt metal doesn't necessarily know how to make a good weld.

3) The code has lots of rules on weld joint design, for example in UG-34 and UW-16. It is common for novices to come up with joint designs that appear to keep stresses below allowable values, but would rupture from metal fatigue or crevice corrosion in service.

4) The NDT you choose for your welds will depend on the joint efficiency factors you want to use from table UW-12.

5) Your vessel will need to be hydrostatically tested per UG-99.

6) Appendix 10 is worth a read.

I can't review all the details of your design or guess what you haven't read in the code, so this is nowhere close to a checklist of things you should check. I just hope that some of the point I've suggested above can scare you into realizing how much you don't know.

 

[Jmoore1]

I hope when you said the max temp of the metal was well below what your application was you meant well above :/.

 

[JStephen]

"Demonstrate that maximum stresses are below maximum allowable stress as given in ASME II"

Note that for major items, the code specifies how the stresses are calculated, not just the maximum stress.

On the weld qualification/ material- that could potentially be a real mess. Ideally, you'd find somebody (possibly welding consultant) who actually has experience with that material or something really close to it. It may also be advantageous to select the alloy to allow better weldability, rather than picking a specific alloy with a weld-it-or-bust approach.